Perpendicular recording: opening the doors for 10-fold hard drive capacity expansion

Computer Technology Review, June-July, 2005 by John Best

While the hard drive industry has been using longitudinal recording successfully for five decades, it is now within two product generations of reaching its practical limit.

For about the past decade, scientists and engineers have pondered the potential effects of a natural phenomenon called superparamagnetism and postulated when its presence might interfere with the progress of the hard disk drive (HDD) industry.

Since the first commercial hard drive was introduced in 1956, the industry has grown storage capacity exponentially by decreasing the size of the magnetic grains that make up data bits. In effect, the smaller the magnetic grain, the smaller the bit, the more data that can be stored on a disk. With longitudinal recording, we are getting close to the point where data integrity will be harmed if we continue to shrink the magnetic grains. This is due to the superparamagnetic effect.

Superparamagnetism occurs when the microscopic magnetic grains on the disk become so tiny that random thermal vibrations at room temperature cause them to lose their ability to hold their magnetic orientations. What results are "flipped bits"--bits whose magnetic north and south poles suddenly and spontaneously reverse--that corrupt data, rendering it and the storage device unreliable.

Today, the hard drive industry's ability to push out the superparamagnetic limit is more critical than ever as capacity requirements continue to grow dramatically. This is due, in large part, to the increasing use of hard drives in consumer electronic devices. Consumers wanting to store more music, photos and video are looking to the hard drive industry to pack more and more storage capacity on smaller devices. The superparamagnetic effect on current magnetic recording technologies will make that growth impossible within one to two years.

Thanks to renewed interest in a magnetic recording method first demonstrated more than 100 years ago, there's confidence at in the storage industry that the natural effects of superparamagnetism can be further stalled. That method is called perpendicular recording, which when fully realized over the next five to seven years, is expected to enable a 10-fold increase in storage capacity over today's technology. This would enable, for example, a 60-GB one-inch Microdrive, which is used in MP3 players, personal media players, digital cameras, PDAs and other handheld devices.

Hitachi, earlier this month, demonstrated a perpendicular recording data density of 230 gigabits/square inch--twice that of today's density on longitudinal recording--which could result in a 20 gigabyte Microdrive in 2007.

Perpendicular and Longitudinal Recording: How They Differ

For nearly 50 years, the disk drive industry has focused nearly exclusively on a method called longitudinal magnetic recording, in which the magnetization of each data bit is aligned horizontally in relation to the drive's spinning platter. In perpendicular recording, the magnetization of the bit is aligned vertically--or perpendicularly--in relation to the disk drive's platter.

Perpendicular pole and tail-to-tail (south-pole to south-pole). In this scenario, they want to repel each other, making them unstable against thermal fluctuations. In perpendicular recording, the tiny magnets are standing up and down. Adjacent alternating bits stand with north pole next to south pole; thus, they want to attract each other and are more stable and can be packed more closely. This geometry is the key to making the bits smaller without superparamagnetism causing them to lose their memory.

Perpendicular recording allows hard drive manufacturers to put more bits of data on each square inch of disk space-called areal density or data density--because of magnetic geometry. Moreover, perpendicular recording results in the improved ability of a bit to retain its magnetic charge, a property called coercivity.

Though it departs from the current method of recording, perpendicular recording is technically the closest alternative to longitudinal recording, thus enabling the industry to capitalize on current knowledge while delaying the superparamagnetic effect.

The exact areal density at which the superparamagnetic effect occurs has been a moving target, subject to much scientific and engineering debate. As early as the 1970's, scientists predicted that the limit would be reached when data densities reached 25 megabits per square inch. Such predictions were woefully inaccurate; they did not consider the ingenuity of scientists and engineers to skirt technical obstacles. Through innovations in laboratories at Hitachi GST and other companies, those limits have moved forward dramatically. Today, the highest areal density with longitudinal recording has surpassed 100 gigabits per square inch. However, researchers believe the technology will begin losing its ability to maintain data integrity at areal densities much beyond 120 gigabits per square inch, at which time perpendicular recording will become the dominant magnetic recording technology.